The present invention relates to an LPF (low-pass filter) circuit for suppressing an unwanted subcarrier in an FM stereo demodulation output.
A conventional LPF circuit of this type is shown in FIG. 1. In FIG. 1, a composite stereo signal applied to an input terminal 1 is supplied to one input terminal of an adder 6, one input terminal of an adder 7, a 38 KHz oscillator 3 and one input terminal of a multiplier 4. The output of the 38 KHz oscillator 3 is applied to the other input terminal of the multiplier 4. The output of the multiplier 4 is applied to the other input terminal of the adder 7. The output of the adder 6 is applied to the input terminal of an LPF 8 including an inductance L and a capacitance C. The output of the LPF 8 is provided as a left channel output at a left channel output terminal 10. On the other hand, the output of the adder 7 is supplied to the input terminal of an LPF 9 including an inductance L and a capacitance C, similar to the case of the LPF 8. The output of the LPF 9 is provided as a right channel output at a right channel output terminal 11. The adders 6 and 7, the 38 KHz oscillator 3, the multiplier 4 and the inverter 5 form a stereo demodulator 2.
In the circuit thus constructed, the composite stereo signal applied to the input terminal 1 is represented by: EQU (L+R)+(L+R)+(L-R) sin .omega..sub.c t, (1)
where .omega..sub.c is the carrier frequency. In order to form the right and left signals R and L by the use of the composite signal, the 38 KHz subcarrier is multiplied by the composite signal. In the 38 KHz oscillator, a PLL or the like is employed to form the 38 KHz subcarrier by using a 19 KHz pilot signal included in the composite signal. When the output sin .omega..sub.c t of the 38 KHz oscillator 3 is multiplied by the composite signal, the following expression is obtained: EQU (L-R)+(L+R) sin .omega..sub.c t-(L-R) cos 2.omega..sub.c t (2)
The sum of the expressions (1) and (2) is the output of the adder 6, which can be represented by the following expression: EQU 2L+(3L+R) sin .omega..sub.c t-(L-R) cis 2.omega..sub.c t (3)
This output of the adder 6 is applied to the filter 8, whose cut-off frequency is 15 KHz, as a result of which the left channel signal is provided at the output terminal 10.
The difference between expressions (1) and (2) is the output of the adder 7. Therefore, the output of the adder 7 can be represented by the following expressions: EQU 2R+(-L-3R) sin .omega..sub.c t+(L-R) cos 2.omega.ct (4)
As in the above-described case, the output of the adder 7 is applied to the LPF 9, as a result of which the right channel signal is provided at the output terminal 11.
As described above, the conventional circuit for removing the unwanted subcarrier from the output uses LPFs which are made up of capacitors and inductors. With this construction, the signal is deteriorated when passing through the LPF. The deterioration of the signal is attributed mainly to magnetic distortion caused by the core material of the inductor in the LPF.